Reversible pressure-induced first-order phase transitions in anion-excess fluorite KY3F10 studied with single-crystal X-ray diffraction

摘要

KY_3F_(10) (Fm3m Z = 8) has been studied with single-crystal X-ray diffraction to 9.5 GPa in a diamond anvil cell under hydrostatic conditions. At atmospheric pressure, it is an anion-excess 2×2×2 superstructure of fluorite with two building blocks Y_6F_(36) and Y_6F_(32). In the Y_6F_(36) block, six square YF_8 antiprisms share their corners to form an empty F_(12) cubooctahedron. In the Y_6F_(32)block, six square YF_8 antiprisms share their edges to form an empty F_8 cube. Upon compression, there occur phase transitions between 5 and 7 GPa due to an instability of the empty F_(12) cubooctahedra in the 2×2×2 superstructure. At about 7 GPa, the cubooctahedra collapse completely and all the cations in the new structure (Pm3m, 1 = 1} are surrounded by cubes of the F atoms. The excessive fluorine is located in the F_8 cubes. At hydrostatic conditions, all the transformations are reversible. These observations imply that at high pressures the anion-excess fluorite superstructures in general tend to transform to the CaF_2 structure.